Breast imaging is considered the most demanding of medical imaging procedures. With regard to spatial resolution, breast imaging specialists are now commonly interested in imaging lesions or masses that may require an imaging aperture that is about 50 microns in size or less. Contrast requirements are also demanding because lesions or masses to be visualized sometimes have x-ray absorption characteristics similar to that of the surrounding tissue. In this regard, 12 bit contrast resolution, corresponding to about 4000 distinguishable shades between black and white in the resulting image, is often desired.
X-ray mammography is the most sensitive breast imaging modality currently available and is widely used in detecting and diagnosing the nature of small non-palpable breast lesions. Both film-based and digital systems are currently available for breast imaging. In film-based systems, x-rays are transmitted through the patient's breast and impinge upon a phosphor screen. Light emitted from the phosphor screen as a result of the absorption of x-rays is detected by light sensitive film. The film is then developed to yield an image of the patient's breast which can be viewed on a light box. In digital systems, a radiation receiver is used in place of the film. The receiver yields an electronic signal which can be digitally processed for viewing on a high resolution monitor. Currently, only limited field of view digital systems, e.g., 5 cm by 5 cm field of view systems, approximate film based systems in mammographic performance.
Heretofore, film-based systems have been most commonly used for breast imaging and improvements over the years in film-based, x-ray imaging technology have resulted in improved imaging capability and reduced radiation dosage. Film based systems are, however, subject to certain limitations. For example, film granularity and film screen noise limits the spatial resolution of the resulting image. Moreover, films which provide higher resolution images generally require greater radiation doses. In addition, in film-based systems, the resulting image contrast can be significantly affected by scattered radiation. Although the effects of scattered radiation may be reduced by using an antiscatter grid, grids necessitate a greater radiation dosage. Furthermore, the time required to develop film images renders film-based systems less desirable for some applications.
Recently, researchers have recognized that digital imaging systems offer potential advantages over film-based imaging systems. In particular, digital imaging systems avoid the problems of film granularity and film screen noise and are theoretically capable of providing outstanding image resolution. Additionally, in digital imaging systems, once the receiver imaging data has been stored, various processing and display parameters can be manipulated to optimize the displayed image. Digital systems also allow for substantially real-time imaging as may be desired. The stored digital imaging data can also be downloaded for transmission within a computer network and retrieved at remote workstations thereby facilitating information storage, consultation and computer image analysis.
However, current and proposed digital imaging systems for use in mammography do not fully meet the needs of the industry. Digital imaging has been effectively used in the context of stereotactic localization of breast lesions for subsequent needle biopsy procedures. However, such systems are not intended for full field breast imaging and are normally used only to image a relatively small area of the breast where the lesion is located, e.g., a 5 cm by 5 cm window. Expanding such digital imaging systems for full field, single exposure breast imaging would be complicated and expensive due to the high degree of spatial resolution required for imaging small, non-palpable breast lesions. Moreover, such a full field, single exposure imaging system would be affected by scattered radiation. That is, the receiver of such a system would receive significant scattered radiation from the patient's breast in addition to the desired, image-forming x-ray signal. Additionally, use of a grid to reduce the effects of such scattered radiation would necessitate a greater dosage. As a result, subtle breast lesions could be obscured or difficult to ascertain, even with digital processing and display enhancements.